1. Field of the Invention
The present invention relates to a projection display apparatus such as a projection television apparatus and a projection lens device suitable for use therein, and in particular, relates to those in which an original image displayed upon a liquid crystal panel is expansively projected so that the apparatus as a whole can be made in compactness, with a little distortion in picture, as well as being suitable for improvement of brightness, focus and contrast thereof.
2. Description of Related Art
Accompany with diversity of image sources, projection display apparatuses have been widely spread on market because of the commercial values in light weight, low price and compactness thereof. Among of those, following with a remarkable improvement in definition and vignetting factor in a liquid crystal panel, a set of a projection type image display device, in which the liquid crystal panel is used as an original image source to be projected, begins to be brought on market in recent years. This projection type image display apparatus is so constructed that the original image displayed on the liquid crystal panel is enlarged by a projection lens on a screen with full colors.
As an optical system for the projection type image display apparatus, there are already known two types, in one of which, as is disclosed in FIG. 19 of Japanese Patent Laying-Open No. Hei 9-96759 (1997), three (3) pieces of the liquid crystal panels are used (three-plate type), and in the other of which, as is disclosed in FIG. 1 of Japanese Patent Laying-Open No. Hei 4-60538 (1992), a single (1) piece of the liquid crystal panel is used(singular-plate type). However, in those days, in view of being small in the number of parts and of cost reduction thereof, the optical system of the singular-plate type has been extensively developed.
Construction in the optical system of this singular-plate type (using a single piece of the liquid crystal panel) will be explained by referring to FIGS. 22 and 23. As is shown in FIG. 22, a light beam emitting from a white light source 28 of a metal halide lamp (other than that, a xenon lamp or a halogen lamp can be used) is reflected by a reflection mirror 29 so as to be effectively irradiated upon a condenser lens 27, then obtaining a white light beam being almost in parallel by a collimator lens 26.
In a front of the collimator lens 26 are positioned three kinds of dichroic mirrors 23, 24 and 25. Each of those dichroic mirrors 23, 24 and 25 has a characteristic of selectively reflecting a light of wavelength of green, red or blue, respectively, while penetrating others therethrough. R, G and B indicate the lights of red color, green color and blue color, which are separated by those dichroic mirrors 23, 24 and 25, respectively. In this conventional art, it is so constructed that the blue color light beam and the green color light beam enter into the liquid crystal panel 22 diagonally as a standard of the red color light beam.
On the liquid crystal panel 22, there are provided pixels corresponding to the three primary colors, i.e., red, green and blue, and each pixel is so set that it can obtain a light transmittance corresponding to the level of luminance signal in a picture signal, respectively. Therefore, the lights of the red, green and blue are modulated meeting with the picture signal level so as to obtain a desired picture on the liquid crystal. This picture which is displayed on the liquid crystal 22 is enlarged and projected on the screen 20 by the projection lens device 21.
In order to take the picture light emitted from the liquid crystal 22 into the projection lens device 21 effectively, there was proposed an optical system in which a convex lens showing condensing function is provided between the liquid crystal panel 22 and the projection lens device 21 (not shown in FIGS. 22 and 23).
The white light source 28 generates heat by itself and the liquid crystal panel (including a polarization plate thereof) is heated due to absorption of the incident light thereon, therefore they come to be reasons of causing damages thereon, respectively. Then, for reducing the increase of the temperature of them, a cooling fan (not shown in the figure) is provided to compulsively cool them, so that they can be used within a predetermined range of the temperature.
FIG. 23 shows an another example of the optical system of the conventional singular-plate type (using a single liquid crystal panel). In this, the same reference numerals are attached to the same elements in the optical system shown in FIG. 22. A light beam emitting from the white light source 28 of the metal halide lamp (other than that, the xenon lamp or the halogen lamp can be used) is reflected by the reflection mirror 29 so as to be effectively irradiated upon the condenser lens 27, then obtaining the white light beam being almost in parallel by the collimator lens 32. Then, by means of a polarization beam splitter 31 (hereinafter, it is abbreviated with xe2x80x9cPBSxe2x80x9d), a S-polarization light and a P-polarization light are combined to each other. Thereafter, the white light which is converged by the condenser lens 30 is converted into a light which is almost in parallel through the collimator lens 26 to be entered into the three kinds of dichroic mirrors 23, 24 and 25 in a front thereof. Those dichroic mirrors 23, 24 and 25, the liquid crystal panel 22, the projection lens device 21 and the screen 20 have the same functions as in the conventional example shown in FIG. 22, therefore, the detailed explanations of those are omitted here.
On a while, the compulsive cooling of the white light source 28 and the liquid crystal panel (including the polarization plate thereof) is also same to that shown in FIG. 22, therefore, the detailed explanation of it is omitted here.
For realizing an image display device of a projection type being compact in size and of a rear type with using the optical system shown in FIG. 22 or 23, it is essential to shorten the projection distance (i.e., the distance from the projection lens device to the screen), and a wide-angle projection lens device is necessitated.
At this moment, if an ordinal wide-angle projection lens device is used, a ratio of light amount at a peripheral portion decreases down because of a characteristic in distribution of light by the liquid crystal panel. The reason of this will be given in detail. In the optical system of the singular-plate type (using only one piece of the liquid crystal panel), as shown in FIG. 22 or 23, since the dichroic mirrors 23, 24 and 25, which are positioned between the liquid crystal panel 22 and the white light source 28, vary the respective spectrum transmittance and reflectance thereof depending on the incident angles, therefore, the light beam from the white light source 28 comes to be almost in parallel to be radiated upon the liquid crystal panel 22. Also there was already known an apparatus, in which a micro-lens is provided on a light incident surface of the liquid crystal panel 22 for a purpose of increasing the vignetting factor thereof (hereinafter, an explanation will be given on the liquid crystal panel on which the such micro-lens is provided).
A principle light beam of luminous or light flux incident upon the projection lens device 21 from respective object points on the liquid crystal panel 22 comes to be almost in parallel to an optical axis of the projection lens device due to the reason mentioned in the above, and an expansion angle of it is proportional to an aperture number of the micro-lens. If an ordinal projection lens device of wide-angle is used in such the optical system, the light flux which is incident upon the projection lens device 21 from the periphery of the liquid crystal panel is extremely reduced down, thereby the peripheral portion of the enlarged picture projected upon the screen 20. Furthermore, for the projection lens device which is used in the optical system of the image display device of projection type, there are remained many problems to be solved, including followings:
(1) ensuring high focus quality at every corner of the screen;
(2) reducing a F number for purpose of increase in the brightness of the screen;
(3) reducing distortion, since adjustment in the convergence is impossible; and
(4) reducing reflection upon the lens surface and suppressing loss of the brightness, thereby ensuring sufficient contrast performance.
On the other hand, in the optical system for projection using the liquid crystal panel according to the conventional art, there are provided cooling fans (not shown in the figure), each one for cooling the white light source 28 or the liquid crystal panel (including the polarization panel) respectively. Therefore, it causes a reason of cost-up of it. Further, reduction of wind noises which are generated by the cooling fans also comes to be a problem to be solved.
An object of the present invention is, for dissolving the problems mentioned in the above, to provide a projection display apparatus and a projection lens device for use therein, with which the apparatus can be made compact in size, and being suitable to obtain a picture of low image distortion, as well as improving brightness, focus and contrast thereof.
For dissolving the above-mentioned object, in accordance with the present invention, there is provided a projection lens device for projecting an original image being displayed on an image source upon a screen, comprising in sequence from the screen:
a first lens group having a negative refractive power as an entire system thereof;
a second lens group having a positive refractive power as an entire system thereof; and
a third lens group having a negative refractive power as an entire system thereof. With such the construction, it is possible to obtain a flat picture image for a wide field angle of 80 degree, therefore, obtaining good focus quality at every corner of the screen. Further, with this construction, since the first lens group and the second lens group of the negative refractive power are positioned symmetrically at both sides of the second lens group of the positive refractive power, thereby it is also possible to suppress distortion at low level.
However, conventionally, the projection lens device of such the construction has a drawback that the first and the third lens groups comes to be large in the lens diameters thereof, thereby increasing production cost thereof. Then, in accordance with the projection lens device of the present invention, the third lens group includes a lens, which has a negative refractive power (with diverging function) of aspherical surface in the vicinity of the optical axis and has a positive refractive power (with condensing function) at periphery portion thereof, thereby reducing the lens diameter as small as possible while maintaining the advantages of the basic structure mentioned in the above.
Further, with provision of an aspherical lens having a positive refractive power (with condensing function) in the vicinity of the optical axis thereof and having a negative or no refractive power (with or almost without diverging function) at periphery portion thereof, and combining it with the aspherical lens of the third lens group mentioned above, it is possible to let it to have a function of a beam expander optical system (with converting the width of luminous flux), which can compress the luminous flux from the liquid crystal panel in the radial direction thereof. As a result of this, since it is possible to decrease the effective height of the object surface, therefore, correction of the aberrations including chromatic aberration of magnification becomes easy. Further, with the projection lens device of such the construction, since the size of an exit pupil, through which the luminous flux forming an image at periphery of the screen passes, is larger than that of the exit pupil on the optical axis thereof, and also since it is a telecentric structure in which a main light of the luminous flux is almost in parallel to the optical axis of the projection lens device, it is possible to ensure sufficient light amount ratio at the periphery. Further, for realizing high focus on entire screen and for obtaining bright picture, in the projection lens device according to the present invention, the aspherical lens is provided at a position where the light flux forming an image at a center of the screen and the light flux forming an image at the most peripheral portion thereof do not overlap each other. With the aspherical lens, a glass one is expensive, therefore an aspherical lens made of plastic material is used. However, the plastic aspherical lens has a significant problem that it fluctuates the refractive power due to changes in refractive index and shape thereof, which is caused by change of temperature or hygroscopic swelling thereof. Therefore, (1) the plastic lens is so formed that it has a thickness as uniform as possible, thereby reducing the fluctuation in the refractive power due to the changes in the refractive index and the shape caused by change of the temperature or the hygroscopic swelling. (2) It is so constructed that the fluctuation in the refractive power, which is obtained from a local shape of the plastic aspherical lens due to changes in the temperature and humidity, is canceled by combining a plurality of the plastic aspherical lenses.
For realizing the high contrast with reducing reflection upon a lens surface, in the projection lens device according to the present invention, loss by the reflection of an image light is reduced by means of optically connecting between the projection lens device and the liquid crystal panel through a medium having refractive index Nd (refractive index with respect to a light of wavelength 587.6 nm) being greater than 1.2. In more concrete, a cooling liquid (medium having the refractive index Nd (refractive index with respect to a light of wavelength 587.6 nm) being greater than 1.2) is fulfilled within a space defined between the projection lens device and the liquid crystal panel, thereby reducing the loss due to the reflection of the image light and enabling the picture of high contrast as well. Further, the liquid crystal panel and the polarizing plate have tendency to decrease the polarization characteristics thereof with increase of the temperature (for instance, at 70xc2x0 C.), thereby causing the decreasing of the contrast characteristic of the display apparatus. Therefore, according to the present invention, both of those, i.e., the liquid crystal panel and the polarizing plate are cooled by means of a cooling liquid in order to obtain an image of high contrast.